This invention relates to a paper feed control system for a page printer such as a cut sheet-double side printing laser printer which carries out continuous full-page printing.
First, the operation of a conventional cut sheet-double side printing laser printer will be described with reference to FIG. 3. FIG. 3 shows the overall construction of the cut sheet-double side printing laser printer according to the prior art. Paper sheets are stacked in a paper feed hopper 1. When printing data is fed from a host computer system to the printer, the printer makes necessary preparations for the printing, such as the rotation of a photosensitive drum and etc., at the same time begins to process the printing data to develop the same into on-off information ( dot information) of the laser. When the printing preparations are completed after the dot development is finished, a paper feed roller 3 is driven, and an uppermost one of the paper sheets stacked in the paper feed hopper 1 is fed therefrom. The paper sheet thus fed from the paper feed hopper 1 is fed through feed rollers 5 to 8 into a printing mechanism 9. A toner image transferred onto the paper sheet in the printing mechanism 9 is fixed at a fixing device 10. In the case of a one-side printing mode, the paper sheets thus subjected to the fixing are sequentially discharged into a paper stacker 11, and are stacked therein. The type of stacker, in which at this time the paper sheets are stacked in such a manner that the printed results are arranged in the order of the printing with their printed surfaces directed downward, as illustrated, is commonly called a face down stacker.
In the case of a double-side printing mode, a gate switching operation is effected immediately before the paper which has been sheet subjected to a one-side printing arrives at a path switching gate 12, and the paper sheet is sent to a switch back portion 13. The leading and trailing edges of the paper sheet are reversed at the switch back portion 13, and the paper sheet is sent into a return hopper 14 via a return feed path. The paper sheet is stopped for a moment at the return hopper 14, and waits until the dot development of the reverse surface printing data is completed. When the dot development is completed, a return paper feed roller 15 is driven, and the paper sheet is sent out of the return hopper 14, and again passes through the printing mechanism 9 and the fixing device 10, so that the reverse surface data is transferred and fixed. The paper sheet subjected to the double-side printing is discharged to the paper stacker 11, so that the inverted stacking is effected as described above for the one-side printing mode. Namely, in the case of the double-side printing mode, the stacking is carried out in such a manner that the printing result obtained secondly is disposed at the lower side to provide the first page. Therefore, in the double-side printing mode, generally, the printing order is changed in the printing control device in such a manner that the even number page is first printed, and then the odd number page is printed.
As shown in the drawings, the return feed path needs to have a length at least corresponding to the length of one paper sheet, and usually has a length corresponding to the total length of 4 to 10 paper sheets because the fixing device, a resist mechanism, etc., are provided at the return feed path. Therefore, when the front and reverse surfaces of one paper sheet are to be continuously printed, the waiting time from the end of the printing of the even number page to the start of the printing of the odd number page corresponds to a time period during which several pages can be printed. Thus, the printing speed of the printer is greatly lowered. Therefore, it is a common practice that during the time when the paper sheet whose even number page has been printed is traveling along the return feed path, a subsequently-fed paper sheet is printed if the printing data for this subsequent paper sheet has been received.
Namely, when the printing is to be continuously carried out in the double-side printing mode, the even number page (second page) of a first paper sheet is first printed, and then before the first page of this first paper sheet is printed, subsequent fourth and sixth pages are printed, thereby preventing the total printing speed from being lowered.
However, this means that even when the printing data is received from the host device, the printer can not carry out the printing immediately and that it is necessary to temporarily store the data for several pages by a storage means such as a memory.
A conventional method of decreasing the capacity of the memory for storing the dot data after the dot development is to effect the dot development in the order of printing of the pages.
The dot development time for one page varies depending on the printing data, and usually graphic data requires a longer time than character data. Therefore, when the time required for the dot development of the odd number page is longer than the time required for the return of the paper sheet, the paper sheet is caused to stand by at the above return hopper portion to wait for completion of the dot development.
Here, in the above case, that is, during the time when the printing is carried out in the order of the 2nd, 4th, 6th, 1st, 3rd and 5th pages, let's assume that the dot development for the 1st page requires a relatively long time. The three paper sheets are fed from the paper feed hopper 1, and the 2nd, 4th and 6th pages are printed, and the three paper sheets are sequentially sent to the return hopper 14 via the return feed path.
Namely, when the dot development time for the 1st page is long, all three paper sheets are stacked in the return hopper portion. Therefore, the return hopper 14 must be able to stack a plurality of paper sheets, and also must again feed the stacked paper sheets one by one in the order of stacking.
Incidentally, when considering a stable travel (high-precision travel without jamming) of paper sheets which is the most important of all the basic performances of the printer, paper feed problems conventionally occurs most frequently when the paper sheets are separately fed one by one from the paper stack portions (i.e., the paper feed hopper 1 and the return hopper 14).
Namely, at the other portions, the paper sheet already separated is fed by the upper and lower rollers. In contrast, the paper sheets are stacked in intimate contact with adjacent ones in the paper feed hopper 1 and the return hopper 14, and therefore there is a high possibility that two or more paper sheets may be fed simultaneously therefrom. Furthermore, because of the friction feed applied to only one surface of the paper sheet, the paper sheet may greatly slip, which results in problems that the printing position is displaced and that the paper sheet is liable to become jammed. Particularly in the case of the return hopper 14 in the above double-side printing device, static charges develop on the paper sheet at the time of the one-side printing, and therefore the paper sheets can not be easily separated front one another.
In order to overcome double sheet feeding due to incomplete paper separation and the paper feed problems due to the paper slip, various paper separation mechanisms and slipless paper feed mechanisms have heretofore been proposed.
However, none of them have been 100% effective because of the limitation on the overall size of the printing device and other reasons.